A passive optical network (PON) is a network architecture employing fiber cables from a central office to local premises. It employs passive optical components to enable a single optical feeder fiber to serve multiple premises. A PON consists of a central office node, where the optical line terminal (OLT) equipment is located, one or more termination nodes at customer premises, called optical network terminations (ONT) or optical network units (ONU) and further infrastructure such as fiber, power splitters (PSs), filters, etc. which connect the central office node to the termination nodes. This infrastructure is called the optical distribution network (ODN). In other words, the ODN provides the optical transmission medium from the OLT towards the ONUs and vice versa. The ODN comprises a plurality of optical links and passive optical components arranged so as to form a point to multipoint structure whose root is connected to the OLT. In a passive optical network, a single optical fiber, referred to as feeder fiber and acting as an optical path, guides the light towards the remote node (RN) where it is delivered to the different drop sections by means of data splitters, filters, or any other passive equipment. More specifically, the optical path may split at an optical splitter or power splitter into several branch paths, with each branch path connected to a single ONU by a port of the splitter. From the RN the light is guided through the optical path towards the customer premises which are considered as endpoints. These are referred to as ONT if the unit serves one single endpoint or ONU if the unit serves multiple endpoints. On the uplink, the ONT/ONU sends user data back to the OLT using the same or a different wavelength. With the term uplink or upstream we refer to signals travelling from the ONUs to the OLT. With the term downlink or downstream we refer to signals travelling from the OLT to the ONUs.
The optical attenuation caused by the various components of the ODN adds up to values, which nowadays are bridged by transmitters and receivers from the endpoints of this ODN. Typical performance data for 10 Gbit/s are a reach of up to 40 km and split up to 64. There is a need to extend the reach and split factors in ODNs, especially due to the introduction of long reach PONs. Today the reach and split ratio of an ODN can be extended by inserting an optical amplifier (OA) in the network. The most usual position of the OA is in the remote node (RN) after the typically 60-90 km feeder line and before the power splitter (PS) of 128-512 and 0-10 km drop fiber. In the current state of the art an optical amplifier for each direction (downstream and upstream) is added.
To have optical amplification in the RN has the disadvantage that the OA needs to have access to an electrical power supply in order to operate. As the existing RN is based on passive components, it typically does not have access to electrical power.